|This Vector tutorial has been selected by PSIgate as a recommended teaching tool. Click the PSIgate logo to access their large inventory of Science Tutorials.|
In this tutorial we will examine some of
the elementary ideas concerning vectors. The reason for this introduction
to vectors is that many concepts in science, for example, displacement,
velocity, force, acceleration, have a size or magnitude, but also they
have associated with them the idea of a direction. And it is obviously
more convenient to represent both quantities by just one symbol. That is
|Graphically, a vector is represented by an arrow, defining the direction, and the length of the arrow defines the vector's magnitude. This is shown in Panel 1. . If we denote one end of the arrow by the origin O and the tip of the arrow by Q. Then the vector may be represented algebraically by OQ.||
This is often simplified to just . The line and arrow above the Q are there to indicate that the symbol represents a vector. Another notation is boldface type as: Q.
Note, that since a direction is implied, . Even though their lengths are identical, their directions are exactly opposite, in fact OQ = -QO.
The magnitude of a vector is denoted by absolute value signs around the vector symbol: magnitude of Q = |Q|.
The operation of addition, subtraction and multiplication of ordinary algebra can be extended to vectors with some new definitions and a few new rules. There are two fundamental definitions.
| #1 Two vectors, A and
are equal if they have the same magnitude and direction, regardless of
whether they have the same initial points, as shown in
|#2 A vector having the same magnitude as A but in the opposite direction to A is denoted by -A , as shown in Panel 3.||
|We can now define vector addition. The sum of two vectors, A and B, is a vector C, which is obtained by placing the initial point of B on the final point of A, and then drawing a line from the initial point of A to the final point of B , as illustrated in Panel 4. This is sometines referred to as the "Tip-to-Tail" method.||
The operation of vector addition as described here can be written as C = A + B
This would be a good place to try this simulation on the graphical
addition of vectors. Use the "BACK" buttion to return to this point.
|The graphical representation is shown in Panel 5. Inspection of the graphical representation shows that we place the initial point of the vector -B on the final point the vector A , and then draw a line from the initial point of A to the final point of -B to give the difference C.||
Associative Law for Addition: A + (B + C) = (A + B) + C
|The verification of the Associative law is shown in Panel
If we add A and B we get a vector E. And similarly if B is added to C , we get F .
Now D = E + C = A + F. Replacing E with
(A + B) and F with (B + C), we get
(A +B) + C = A + (B + C) and we see that the law is verified.
Stop now and make sure that you follow the above proof.
Associative Law for Multiplication: (m + n)A = mA + nA, where m and n are two different scalars.
Distributive Law: m(A + B) = mA + mB
These laws allow the manipulation of vector quantities in much the same
way as ordinary algebraic equations.
| Let us consider the two-dimensional (or x, y)Cartesian
Coordinate System, as shown in
|We can define a unit vector in the x-direction by or it is sometimes denoted by . Similarly in the y-direction we use or sometimes . Any two-dimensional vector can now be represented by employing multiples of the unit vectors, and , as illustrated in Panel 8.||
The vector A can be represented algebraically by A = Ax + Ay. Where Ax and Ay are vectors in the x and y directions. If Ax and Ay are the magnitudes of Ax and Ay, then Ax and Ay are the vector components of A in the x and y directions respectively.
|The actual operation implied by this is shown
in Panel 9.
Remember (or ) and (or ) have a magnitude of 1 so they do not alter the length of the vector, they only give it its direction.
It is perhaps easier to understand this by having a look at an example.
|Consider an object of mass, M, placed on a smooth
inclined plane, as shown in Panel 10. The gravitational force acting on
the object is
F = mg where g is the acceleration due to gravity.
In the unprimed coordinate system, the vector F can be written as F = -Fy, but in the primed coordinate system F = -Fx'+ Fy'. Which representation to use will depend on the particular problem that you are faced with.
For example, if you wish to determine the acceleration of the block
down the plane, then you will need the component of the force which acts
down the plane. That is, -Fx'which
would be equal to the mass times the acceleration.
|By resolving each of these three vectors into
their components we see that the result is Panel 11.
Dx = Ax + Bx + Cx
Dy = Ay + By + Cy
Now you should use this simulation to study the very important topic
of the algebraic
addition of vectors. Use the "BACK" buttion to return to this point.
Very often in vector problems you will know the length, that is, the magnitude of the vector and you will also know the direction of the vector. From these you will need to calculate the Cartesian components, that is, the x and y components.
|The situation is illustrated in Panel 12. Let us assume that the magnitude of A and the angleq are given; what we wish to know is, what are Ax and Ay?||
From elementary trigonometry we have, that cosq
= Ax/|A| therefore Ax = |A| cos q,
Ay = |A| cos(90 - q) = |A| sinq.
|In Polar coordinates one specifies the length of the line and it's orientation with respect to some fixed line. In Panel 13, the position of the dot is specified by it's distance from the origin, that is r, and the position of the line is at some angle q, from a fixed line as indicated. The quantities r and q are known as the Polar Coordinates of the point.||
It is possible to define fundamental unit vectors in the Polar Coordinate
system in much the same way as for Cartesian coordinates. We require that
the unit vectors be perpendicular to one another, and that one unit vector
be in the direction of increasing r, and that the other is in the direction
of increasing q.
|In Panel 14, we have drawn these two unit vectors with the symbols and . It is clear that there must be a relation between these unit vectors and those of the Cartesian system.||
|These relationships are given in Panel 15.||
And secondly, the vector or cross product of two vectors, which results in a vector.
In this tutorial we shall discuss only the scalar or dot product.
|The scalar product of two vectors, A and B denoted by A·B, is defined as the product of the magnitudes of the vectors times the cosine of the angle between them, as illustrated in Panel 16.||
The rules for scalar products are given in the following list, .
And in particular we have , since the angle between a vector and itself is 0 and the cosine of 0 is 1.
Alternatively, we have , since the angle between andis 90º and the cosine of 90º is 0.
In general then, if A·B = 0 and neither the magnitude of A nor B is 0, then A and B must be perpendicular.
The definition of the scalar product given earlier, required a knowledge of the magnitude of A and B , as well as the angle between the two vectors. If we are given the vectors in terms of a Cartesian representation, that is, in terms of and , we can use the information to work out the scalar product, without having to determine the angle between the vectors.
Because the other terms involved,, as we saw earlier.
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